One of the most water-intensive industries in the United States is the processing of phosphate rock mineral. A major part of this industry is located in Florida which processes over 80% of the mineral processed in the whole country. During such processing extraneous, undesirable material in the ore, e.g., clay and slica, accumulates in the form of an unsettable very fine suspension in the water. This water is employed for washing and flotation-concentration of the crushed ore. The flotation tailings typically consisting of 150 mesh sand easily dewater. The clay slime, however, does not settle easily. The slime is stored in earthen dams where the initial 4 to 5 percent solids becomes 15 to 25 percent over several years of settling, and eventually to 30 to 55 percent solids in the course of 20 to 30 years. The phosphate values in the slime cannot be recovered because of the difficulty of initial dewatering and the high cost of chemicals required for successful flotation.
Thus, very large tonnages of water drawn from natural sources are held up as slime and cannot be immediately reused. Phosphate mining has already lowered the fresh water table in some places like the Polk Country.
Several Governmental and Industrial Research Organizations are engaged in establishing economical methods to dewater the Florida phosphate industry's slimes. The techniques developed so far include flocculation and consolidation of the particles by addition of various electrolytes, flocculation and consolidation by techniques involving freezing and thawing and electroosmosis, with or without filtration. So far it cannot be said that any one of the methods investigated has met with complete success.
Other techniques under development include chemically hardening phosphatic clays by the addition of lime or gypsum to the clays. In addition, the use of accelerators or surfactants in combination with lime or gypsum are being tested. See Florida Institute of Phosphate Research, Information Clearinghouse Newsletter; Volume V, Number 2; Summer 1984. C. C. Cooke et al., U.S. Pat. Nos. 3,761,239(1973) and 3,763,041(1973) describes a method for making a land-fill composition from slimes. This method involves mixing the slimes with tailings. Large amounts of tailings are used, preferably between 60 and 99 percent by weight based on the weight of the slimes. This admixture of slimes with tailings dewater at a substantially greater rate than is achieved using gravity settling. Optionally, after the slime is dewatered calcium sulfate may be incorporated into the reconstituted landfill compositions to produce a fertile landfill having acceptable bearing strength. A method has now been discovered for dewatering phosphate slimes which involves adding calcium sulfate to the slimes, but which does not require the addition of tailings. The method is surprisingly effective in permitting the separation of the liquid and solid components of the slimes and allows for recovery of dissolved mineral values.